The present invention relates to determination of waking and sleep stages experienced by a resting subject from whom various physiological data are obtained and, more particularly, determining, through an operator viewing a display of representations of such data as a basis for setting criteria, waking and different stages of sleep.
Substantial research has been undertaken directed toward understanding the nature of sleep and sleep disorders. This research has yielded considerable information concerning human patterns of sleeping and not sleeping, the physiological activities occurring in humans during sleep including sleep structure as to type, and various cognitive aspects of sleeping, such as dreaming. In addition, substantial information has been obtained concerning various sleep disorders.
In assessing the physiological activity occuring during sleep, various kinds of signal data are obtained, recorded and analysed. A primary signal which is obtained yields the electroencephalographic signal data which is used as a reference with respect to which sleep events and other signals obtained are correlated. The other kinds of signals which are almost always obtained in sleep research, or in clinical evaluations, are electromyographic signals and electrooculographic signals. In addition, other often obtained signals are electrocardiac signals, respiration signals, blood oxygen saturation signals, and the like.
In reviewing and evaluating the data obtained from such signals, researchers have come to the conclusion that sleep is structured in five different characteristic patterns or stages when viewed against the electroencephalographic signal data for normal adults. This has led to a set of rules or descriptions which have become generally accepted as a basis for classifying a physiological signal data record of a sleeping adult into the various characteristic kinds of sleep which have occurred, i.e., "scoring" waking and sleep stages in the physiological signal record.
Though the rules have relatively detailed descriptions of the various sleep stages, the following gives a rather general summary of the stages under the rules of descriptions. The recorded data is viewed as a succession of "epochs," each of which is a data accumulation period of typically 30 seconds, though other periods such as 20 seconds and 60 seconds can also be used. Each such epoch is then scored as being in the waking stage or one of the sleep stages. Frequency ranges in the electroencephalographic signals are classified into the four traditional frequency ranges for such signals, these being the "delta" range of from 0.5 to 2.0 Hz, the "theta" range of from 2.5 to 7.5 Hz, the "alpha" range of from 8.0 to 12.5 Hz, and the "beta" range of 13.0 Hz or greater.
The waking stage and sleep stage descriptions in terms of the physiological signals obtained are then as follows:
Stage W: The waking stage has electroencephalographic signals with frequency content in the alpha range or low voltage, mixed frequency range result, or both. This stage is usually accompanied by a relatively high electromyographic signal level and by rapid eye movements.
Stage 1: This stage has relatively low voltage, mixed frequency electroencephalographic signals, most of which are in the theta frequency range. There is an absence of sleep spindles and K-complexes (both described below). This is also true of rapid eye movements (described below). The electromyographic signal data amplitudes are usually greater than those in Stage REM as indicated below. Epochs between two sleep spindles which are more than three minutes apart and which meet the criteria otherwise for Stage 1 are considered to be Stage 1 epochs, as are those epochs following a movement arousal (described below) between pairs of sleep spindles separated in time by less than three minutes.
Stage 2: The electroencephalographic signal must have sleep spindles or K-complexes, or both, in an epoch but not have a predominance of high amplitude waveform portions with frequency content in the delta frequency range. The electromyographic signal usually has amplitudes above those used in describing Stage REM. If sleep spindles or K-complexes are not present in the epoch, the epoch must be between a pair of sleep spindles or K-complexes not separated in time by more than three minutes, and without any intervening movement arousals or, in an exception for the occurrence of EMG amplitudes consistent with Stage REM, there must be no intervening rapid eye movements. If the sleep spindles or K-complexes are separated by more than three minutes, score the epoch as Stage 1. Occurrence of a movement arousal between pairs of sleep spindles or K-complexes separated by less than three minutes leads to a determination on the basis of the epochs, prior to the movement arousal, being scored Stage 2 epochs while those after the arousal are scored Stage 1 epochs.
Stage 3: The electroencephalographic signal data must have 20%, but not more than 50%, of the epoch having waves in the delta frequency range with amplitudes greater than 75 U V peak-to-peak for that epoch to be considered sleep at this stage. The occurrence or not of sleep spindles or K-complexes has no effect on the determination that sleep is of this stage.
Stage 4: The electroencephalographic signal data in an epoch must have a frequency content in the delta range with an amplitude greater than 75 U V peak-to-peak during 50% or more of that epoch for it to be considered sleep of this stage. Again, the occurrence of sleep spindles or K-complexes has no effect on the determination of sleep in this stage.
Stage REM: The electroencephalographic signal in the epoch is of a relatively low voltage, mixed frequency coupled with rapid eye movements occurring in the vicinity of the epoch and associated with a relatively low electromyographic signal level. An epoch cannot be considered as part of this stage of sleep if more than one-half thereof occurs between two sleep spindles or K-complexes separated by less than three minutes with no intervening rapid eye movements. An epoch cannot be considered part of this stage of sleep either if the last significant movement arousal occurred more recently than the last rapid eye movement.
Sleep spindles, K-complexes, rapid eye movements and movement arousals are transient events, being of a short duration, which occur from time to time during gathering data from physiological signals measured on a subject. A sleep spindle is defined as an electroencephalographic signal waveform event of amplitude changes occurring in the 12.0 to 14.0 Hz range (considered by many to extend to the 11.0 to 15.0 Hz range) of a sufficient peak-to-peak amplitude with the rest of the waveform thereabout of a sufficiently lower frequency content with a time duration exceeding 0.5 seconds. A K-complex is a waveform portion having a negative peak preceding a positive peak with a time duration of these waveform events exceeding 0.5 seconds, with these peaks separated by a sufficient peak-to-peak voltage.
The electrooculographic signals obtained are taken from an electrode provided for each eye. One electrode is located approximately 1.0 cm. above and slightly lateral to the outer canthus of one eye, and the other electrode is located approximately 1.0 cm. below and slightly lateral to the outer canthus of the other eye, both referred to a common ear or mastoid electrode. Because eye movements in Stage REM and waking are binocularly synchronous, this arrangement provides opposite amplitude direction signals from these electrodes for almost all eye movements. Those signals that move together mostly represent electroencephalographic artifact, i.e., electrooculographic signal noise. The difference in the electrooculographic signals at points of sufficient amplitude indicates that a rapid eye movement or REM has occurred.
Movement arousals are found by noting occurrences of a sufficient change in the electromyographic signal data. A movement arousal occurs when such a change happens in conjunction with changes in either the electroencephalographic signal data or the electrooculographic signal data.
Even though the foregoing rules are offered in a summary description (the actual descriptions would require several pages), these descriptions suggest how much must be considered for each epoch being scored by a trained scorer. Since the typical adult night's sleep is seven or eight hours, epochs of 30 seconds duration will lead to a rather large number of epoch records to be considered by such a scorer in allocating the sleep of an adult to the various waking and sleep stages. Thus, a means for easing the tedious effort required of such a scorer would be desirable to reduce the time until the information contained in the acquired data becomes available, to reduce errors in the analysis of the data and to reduce cost.